In the design of conventional pulsed radar systems, the maximum range performance against targets of a preselected type is a function of the pulsed energy transmitted by such radar. As is well understood in the art, such pulsed energy is determined by the product of the transmitted power level and the pulsewidth thereof. Because a given radar transmitter design is usually peak-power limited, a desired energy requirement may be met, ofttimes, only by increasing the pulsewidth or duration of the transmitted pulse. While an increase in the pulsewidth of a given power level increases the maximum range performance of the radar, such pulsewidth increase also limits the range resolution or accuracy to which a range measurement may be made: the larger the pulsewidth, the larger the range resolution or error in the range determination (indicated as the product of pulsewidth and propagation velocity).
Several techniques have been studied for improving the range resolution effect associated with a selected pulsewidth. Such techniques are referred to in the art as pulse compression techniques, and involve means of operating a radar with long pulses to obtain the resolution and accuracy of a short pulse while retaining maximum range or detection capability of a long pulse. By means of such techniques, the transmitted energy pulse is selectively modulated and the receiver is designed to respond to such modulation to compress the received pulse into one of a much shorter pulsewidth.
One such pulse compression technique is referred to as phase-coded pulse compression, in which a long pulse having a preselected duration or pulsewidth is divided into an integer number of subpulses of uniform interval or duration, each interval referred to herein as a bit. The phase of each subpulse interval or bit is then either left unchanged or else reversed in phase by 180.degree. prior to transmission, in accordance with the corresponding position of such bit in a preselected binary code or sequence. Upon reception of the echoes of the phase-coded pulses, the radar receiver reverse-codes the phase-coded waveform and combines the several bit or subpulse intervals thereof, the response to a discrete target (having a radial extent less than that represented by the transmitted pulsewidth) appearing as a high energy return having a compressed pulsewidth.
A discussion, including a full bibliography of phase-coded pulse compression techniques and the selection of optimum coding is provided at pages 497 and 498 of "Introduction to Radar Systems" by Skolnik, published by McGraw-Hill (1962). A particular embodiment of such technique is also taught in copending U.S. application Ser. No. 474,821 filed July 26, 1965, for Phase Coded Pulse Compression Apparatus, by James A. Moulton, assignor to North American Aviation, Inc., whose name has been subsequently changed to North American Rockwell Corporation, assignee of the subject invention.
Such pulse compression techniques, conventionally employing a single pulse compression code, provide a single autocorrelation function having distinct sidelobes. In other words, the non-zero autocorrelation coefficient provided at other than the maximum coefficient (or point of "signal compression") represents a theoretical performance limit which a physical embodiment may approach but cannot exceed.